The architectural design process is often characterised a series of evolving ideas, and involving a cyclical process between design and visualisation. However, the nature of the internal representation still remains unclear. What is actually represented in a designers mental space and what drives and influences the mental design process? If we wish to programme a computer to mimic or work in tandem with the mental processes involved we need to make that representation and the associated cognitive processes explicit. The ways that designers form mental representations are so diverse, personal, and often transient that it is not easy to externalise and articulate them in explicit terms. In order to propose a mental model, we can take in a particular I psychological research approach; that of introspective observation from design drawing . In doing so, we posit an assumption that the designer's drawing can be seen as an extension of the internal mental feature, and hence internal representation could be inferred from the analysis of external representation - the drawing or sketch. This approach contrasts with the protocol analysis approach where mental operations are inferred from words, what could be termed thinking aloud.